Crystal Structure of Magnetoelectric Ba 2 MnGe 2 O 7 at Room and Low Temperatures by Neutron Diffraction

For a symmetry-consistent theoretical description of the ferroelectric phase of Ba MnGe O melilite compound, a precise knowledge of its crystal structure is a prerequisite. Here we report results of single-crystal neutron diffraction experiments on Ba MnGe O at room (300 K) and low (10 K) temperatures. The structural model based on the tetragonal space group P4̅2 m describes the Ba MnGe O symmetry both at room and low temperatures. We found reflections forbidden in the typical P4̅2 m melilite-type structure. A comparison of the experimental data collected by means of both thermal and cold neutrons with simulated multiple diffraction patterns allows us to unambiguously demonstrate that forbidden peaks originate from multiple diffraction (Renninger effect) rather than from real symmetry lowering. The precise structural parameters at 300 and 10 K are presented for the first time and compared with those of other magnetoelectric melilite-type germanates.